The field of the invention is superconducting motors and specifically induction motors of a type commonly known as squirrel cage motors.
Electric induction motors operate by using a magnetic field produced by a stator to induce a current in a rotor. The stator consists of coil windings distributed around the circumference of the rotor. As these coils are excited with an AC current, a magnetic field that varies with time in a sinusoidal fashion is produced. The peak(s) of this magnetic field travels around the circumference of the stator at a rate determined by the frequency of the AC current and the number of poles in the stator.
If there is no load on the motor, the rotor will turn at the same rate that the magnetic field is rotating around the stator. As the load on the rotor increases, the rotor speed will decrease relative to the speed of rotation of the stator field. The difference in the speed of rotation between the rotor and the stator field induces a current in the rotor, since the stator field is no longer fixed with respect to the rotor. This induced current in the rotor creates its own magnetic field that interacts with the magnetic field produced by the stator to produce mechanical forces. The larger the difference in rotational speed between the rotor and the stator field (known as slip), the more current that is induced in the rotor and consequently the greater the torque produced.
One of the most common types of induction motors is known as a squirrel cage motor. This type of motor gets its name because of the configuration of the windings of the rotor. The windings in which a current is induced consist of bars of electrically conducting material running parallel to the axis of the rotor. These bars are short-circuited at both ends of the rotor by conducting rings. The combination of pairs of bars and end rings are the equivalent of one-turn coils. Their configuration resembles the rotating cage in which squirrels and other animals might exercise, hence the name. Squirrel cage motors are sturdy, simple to construct, and of relatively low cost to manufacture.
The efficiency and torque characteristics of squirrel-eage motors can be improved by using superconducting material in the rotor. An immediate consequence of the use of this material is the elimination of the Ohmic resistance of the conductors of the rotor. This would cause the induced current in the squirrel cage to increase. As a result, more torque would be produced for a given power input, i.e. the motor can be made more compact. There is a definite commercial advantage in using high temperature superconductors (HTS) since these materials exhibit superconductivity at temperatures below 90 degrees Kelvin. The cooling system required to operate at this temperature is considerably less expensive than that needed for the low temperature superconductors that have to be cooled down to temperatures below 10 degrees Kelvin. The materials that act as HTS however, are very brittle.
Unfortunately, two problems are encountered when using a squirrel cage rotor constructed with bars of high temperature superconducting material. The first problem is the difficulty of securing electrical contacts at the junction of the bars and end rings of the squirrel cage that preserve the superconducting properties. Furthermore, assuming that a perfect superconducting rotor cage can indeed be constructed, the xe2x80x9ccoilsxe2x80x9d of the squirrel cage rotor would behave as diamagnetic bodies. Hence, they could not be penetrated by the stator magnetic field and no induced current would be able to circulate in the rotor.
The present invention overcomes the two problems mentioned above by finding a simple method of constructing a squirrel cage structure of high temperature superconducting material free of imperfect electrical contacts and by obtaining a means of magnetically linking the superconducting rotor coils to the stator field.
According to the theory of superconductivity, all superconducting materials can be quenched to a normal, non-superconducting state if exposed to a sufficiently strong magnetic field. The value of this xe2x80x9cquenchingxe2x80x9d field varies with the composition of the superconducting material and with the temperature.
Superconducting material that is exposed to a magnetic field stronger then that material""s quenching field value will become non-superconducting and will therefore allow that magnetic field to penetrate it. If a portion of otherwise superconducting material is quenched to the normal state by a strong time-varying magnetic field, a current will be induced in that portion. In the present invention a rotating magnetic field strong enough to quench the superconducting material on the rotor of the present invention is used to induce a current in the superconducting material.
Specifically, the present invention consists of a conventional stator with multiphase coil windings to produce a rotating magnetic field in a manner well known in the art. It is thus one object of the invention to produce a magnetic field that rotates around the rotor at a fixed speed. The present invention also includes a cylindrical rotor comprised of a lightweight central portion surrounded by a ceramic shell. On the outer surface of the shell is deposited a thin film of superconducting material. It is thus another object of the invention to provide a lightweight rotor of superconducting material.
The present invention also includes a cryogenic cooling system to cool the rotor to a temperature below which the outer material becomes superconducting. It is thus another object of the invention to maintain the temperature of the rotor below the critical temperature of the superconducting material.
The superconducting material on the rotor of the present invention is chosen to have a reasonably low quenching magnetic field. One HTS compound that exhibits the desired properties is YBa2Cu3O7. This compound is quenched to the normal state by a relatively weak magnetic field of the order of 1 Tesla. It is thus another object of the invention to allow the superconducting material to quenched to a normal state by a relatively weak magnetic field.
The operation of the present invention was designed to maintain the simplicity and reliability typical of conventional squirrel cage induction motors while providing higher efficiency by reducing resistive losses in the rotor. First, the motor is cooled to a temperature below the critical temperature of the superconducting material. The rotor is thus superconducting at this point. Second, the stator coils are excited with an AC current to produce a rotating magnetic field. The strength of this field is calibrated so that it is strong enough to cause the superconducting material on the rotor to be quenched to a normal state at periodic places. The regions of this normal state are half a pole pitch apart. Since the magnetic field generated by the stator is rotating around the rotor at the synchronous speed, the quenched regions on the rotor also rotate at the same speed if the rotor is prevented from rotating within the stator. The pattern of the quenched spots on the rotor resembles a conventional squirrel cage where the superconducting strips between the quenched regions and the superconducting regions circling the ends of the rotors are the bars and end rings of the squirrel cage. To ensure the formation of the desired pattern, the axial length of the superconducting material must be greater than that of the stator coils.
Once portions of the superconducting material are quenched to a normal state, the stator field can penetrate these regions. This allows a current to be induced in the non-superconducting regions that can then migrate to the superconducting regions. Thereafter the present invention acts as an induction motor. As the motor begins to rotate, the induced current on the rotor xe2x80x9cbarsxe2x80x9d will decline in frequency. Eventually, equilibrium is reached and the current is stabilized at a slip frequency fixed by the load torque.
The foregoing and other objects and advantages of the invention will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention, however, and reference must be made to the claims herein for interpreting the scope of the invention.